1. Field of Invention
The present invention relates to the field of digital communications. More specifically, the present invention provides for a novel quadrature amplitude modulation (QAM) constellation and method of implementation.
2. Background of Invention
The field of data communications typically uses a modem to convey information from one location to another. Modems communicate by modulating a signal carrying digital data, converting the modulated digital signal to an analog signal, and transmitting the analog signal over the transmission medium using various techniques known in the art. Various modulation schemes have been proposed that can effectively increase the information handling capacity of the communication channel. Quadrature amplitude modulation (QAM) has evolved as one of the most attractive schemes to modulate and transmit data over band-limited channels. QAM is a modulation method that is used to encode a variable number of bits into both a phase and amplitude modulated signal.
QAM is accomplished by adding amplitude modulated sine and cosine waves. These two components, 90° out of phase, are said to be in quadrature. By simultaneously modulating amplitude and phase, the signal can carry more bits for every symbol. One convenient way to represent the possible states is to use a constellation pattern diagram where amplitude is represented by the distance away from the intersection of an “I” and “Q” axis. Where “Q” represents the quadrature component of the symbol, and “I” represents the in-phase component. The phase is the angle measured counterclockwise from the 0° reference. By convention, the zero angle reference is normally the positive I axis. The resulting symbol is a complex sinusoidal with a magnitude, phase and frequency. A number of bits are mapped into the in-phase and quadrature components of a complex symbol, which is converted to analog form and then transmitted over a channel. A map of all the possible complex signal vectors is called the constellation of the QAM encoder.
Various QAM constellations are known in the art. When designing a constellation, consideration must be given to:
(1) The minimum Euclidean distance between neighboring symbol points in the constellation, this distance being representative of the noise immunity of the constellation scheme and hence a measure of the bit error rate (BER).
(2) The phase rotation within the constellation affecting the scheme's resilience against clock recovery imperfections and channel phase rotations.
(3) The peak-to-average power ratio, which effects the amplifier requirements of the constellation implementation.
(4) Simplicity of implementation and of detection.
An ideal data communication system requires that the transmission power level be as low as possible, the data rate of the transmission should be as high as possible, and the bit error rate should be as low as possible. In all communications systems, higher data rates and lower bit error rates are desirable. An improvement in any one or two of these three attributes can often lead to a decline in the remaining attributes(s), resulting in a less efficient communication system.
Band-limited channels exhibit limitations on data carrying capacity (bandwidth) due to bounded signal-to-noise ratio (SNR). Under these conditions, lower bit error rates (BER) are obtained by maintaining a greater minimum distance between symbol points of the constellation while at the same time keeping the average power required for the constellation to a minimum. Maintaining a high minimum distance between symbol points lowers the possibility of errors and therefore reduces the BER, however, separating the symbol points also increases the constellation's power level. QAM constellations known in the art attempt to unite the optimal Euclidean distance between symbol points with a given constellation size. For optimal constellations, i.e. those for which the minimum distances between each point and its nearest neighbor(s) are equal, the error rate in high signal-to-noise ratio conditions is mainly determined by the minimum distance.
In a coherent communication system, a receiver requires an absolute carrier reference to detect valid data. Noise and interference can disrupt the carrier reference resulting in phase rotation. When the receiver loses the phase reference, it must reacquire the absolute carrier phase before valid data can again be detected. A non-rotationally invariant communication system must undergo a rigorous absolute carrier phase reference acquisition and reacquisition process, often requiring tens of thousands of unit baud intervals to complete. As such, rotational invariance is advantageous to an efficient communication system.
Various methods are known in the art for establishing rotationally invariant constellations. It is known to establish fully rotationally invariant systems by inserting parity or pilot bits, by shifting the symbol points of a known constellation, and by differential data encoding. However, these known techniques are undesirably complicated, inefficient or applicable only to lower order modulations. By contrast, the present invention provides a rotationally invariant constellation resulting from the geometry of the constellation and associated placement of symbol points.
Various modulation techniques in use today have different associated peak-to-average power ratios. Multi-dimensional modulation schemes, such as QAM, construct the waveform to be transmitted from multiple pulse streams that are generated according to the received data. The pulse shape spans several symbol intervals to minimize the bandwidth of the transmitted waveform and ensure that the transmitted waveform does not interfere with other systems operating at adjacent frequencies. As a result of the required pulse shaping, one data symbol may overlap pulses associated with adjacent data symbols. Certain data sequences will cause these overlapping pulses to add constructively and produce large peaks in the envelope of the transmitted waveform, while other data sequences will result in a cancellation effect and produce small envelope values. This fluctuation in the signal envelope is quantified by the peak-to-average power ratio of the constellation.
Analog front-end amplifiers are used to boost the signal prior to transmission. These amplifiers perform best when the signal envelope remains in the linear region. Large peaks result in non-linear behavior of the amplifier. It is highly desirable to utilize a modulation scheme that exhibits a peak-to-average power ratio (PAR) as low as possible. A low PAR minimizes the demand placed upon the amplifier to handle peak powers that are significantly larger than the average power, allowing the amplifier to operate linearly and also reduce power consumption. Lower peak power and PAR level systems are particularly attractive in wireless communications because they allow the use of less expensive front-end amplifiers, achieve better spectral containment within an allocated frequency band and reduce power consumption.
Various modulation techniques are known in the art to reduce the PAR while still maintaining the data rate and symbol error rate required for high-speed, narrowband communication systems. U.S. Pat. No. 5,606,578 to O'Dea describes a modulation technique utilizing two constellations in an alternating manner to reduce the peak-to-average power ratio of the amplifier. U.S. Pat. No. 5,381,449 to Jasper describes a method by which PAR is reduced through the introduction of additional “pilot symbols” into the constellation. These modulation techniques, and others, have advanced the art and serve to reduce the PAR, however, the current techniques known in the art also result in an undesirable reduction in the data rate of the system and increased complexity.
Advancements are known in the art that describe a PAR reduction scheme via constellation shaping. However, it is observed that constellations generated by these known methods of constellation shaping are neither completely symmetric nor spherical.
There remains a need, therefore, for a QAM constellation appropriate for high data rates that consumes less power, maintains the same bit-error-rate performance, and is easy and cost effective to implement in hardware or software.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.